The types of screens which are designed for screening through gaps in which the surfaces defining the gaps are relatively movable with respect to each other, or so-called gap-screens, are usually designed so as to have a cylindrical housing with an inlet for the injected pulp suspensions and outlets for both the reject and accept portions of the pulp suspensions, respectively. Within these housings, a stator and a rotor are generally located, which between them define one or more gaps through which the pulp suspension is intended to pass. In this manner, coarse material, which cannot pass through the gaps, is separated from the pulp suspensions in the form of a reject.
A gap-screen comprising several gaps can be formed with concentric rings, every second one of which is stationary and thus alternates with rotary such rings. The gaps are then located at alternating spaces with respect to the center of rotation. This, in turn, implies that the flow conditions are different at the different gaps. Furthermore, the number of gaps is restricted by the fact that the diameter of the screen must be limited by factors such as the circumferential speed thereof.
If, instead, the gaps were located radially, i.e., the rings defining the gaps were located sequentially in the axial direction, the diameter of the screen can be limited even with a larger number of such gaps. Problems arise, however, with the mounting and dismounting of the rings defining the gaps, because these rings must be mounted and dismounted individually and sequentially, which becomes both complicated and tedious. At the same time, it is difficult to maintain a definite gap width, because an improper dimension of one ring can affect the size of all of the gaps.